The Federal Communications Commission and cable television testing organizations such as CableLabs have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United states. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG digital coding algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard comprising, for example, high definition television (HDTV) (and standard definition (SDTV)) digital signals for terrestrial broadcasting, in 1994 & 1995. Although the specifics of the standard are yet to be fully determined, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new ATV signals will have to fit into currently unused television channels (so called "taboo" channels) and initially co-exist with conventional analog television signals without co - channel interference. NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
In 1994 and 1995 the FCC will test the so called "Grand Alliance" system, a system which is being cooperatively developed by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992. This system proposes to take the best features from those systems already tested in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has already decided on a video coding algorithm which will comply with the MPEG source coding standards proposed by MPEG (Motion Pictures Experts Group). Specifically, the Grand Alliance intends to comply with MPEG II.
The Grand Alliance HDTV codec achieves a high compression ratio while preserving good picture quality to enable the delivery of high resolution video in a bandwidth-limited channel. The coding algorithm is not lossless and the video compression standard was chosen based on the need to balance high picture quality and compression ratio. Consideration was also given to the need for random access to the coded bitstream and the need for quick recovery from errors in the terrestrial transmission environment.
Although the need for random access and speedy recovery from errors is best satisfied with pure intraframe coding, obtaining good picture quality at the bit rate of about 20 megabits per second (Mbps) for a bandwidth limited terrestrial broadcast channel demands very high compression. This is not achievable with intraframe coding alone and requires a careful balance between intraframe and interframe coding and between recursive and non-recursive temporal redundancy reduction.
In the Grand Alliance system, a number of techniques are used to achieve the high video compression needed. The Grand Alliance compression algorithm, like the MPEG algorithm, uses block-based motion compensation to reduce the temporal redundancy. Motion compensation is used both for causal prediction of the current picture from a previous picture, and for non-causal, interpolative prediction from past and future pictures. The difference signal, which is the prediction error, is further compressed using the quantization of the discrete cosine transform (DCT) coefficients to remove spatial correlation. The DCT coefficients are quantized in an irreversible process that discards the less important information. The quantized DCT coefficients are run length coded to remove zero values as another step of compression. Finally, the motion vectors are combined with the residual DCT information, and transmitted using variable length codes.
Because of the conflict between the requirements for rapid picture acquisition and highly efficient compression, three picture types are defined in the Grand Alliance HDTV/MPEG compression layer.
Intra coded pictures (I-Pictures) are coded without reference to other pictures. They also provide access points to the coded sequence where decoding and picture insertion can begin and are spatially coded with moderate compression.
Predictive coded pictures (P-Pictures) are coded more efficiently using motion compensated prediction from a past I-picture, and a P-picture can also be used as a reference for further prediction.
Bidirectionally-predictive coded pictures (B-Pictures) provide the highest degree of compression but require both past and future reference pictures for motion compensation.
The organization of the three picture types into a coding sequence is very flexible. The choice is left to the encoder and depends on the requirements of the application.
Details of the MPEG and MPEG II coding algorithms are explained in "MPEG: A Video Compression Standard for Multimedia Applications", by Le Gall, Communication of the ACM, vol.34, number 4, Apr. 1991, and ISO/IEC Draft 13818-2 Recommendation H.262, Nov. 15, 1993, which are both incorporated by reference herein.
The powerful and flexible MPEG coding algorithms result in an output bitstream that is both peaky and bursty. One object of the instant invention is to provide a decoder which can process this type of bitstream in an effective and efficient manner.
The structure of the coded bit stream as defined by the Grand Alliance and MPEG utilizes a layered approach as illustrated in Table I. These layers provide for additional local entry points for decoding and program insertion.
TABLE I ______________________________________ LAYERS OF MPEG CODED BITSTREAM ______________________________________ Sequence Layer Group of Pictures Layer (GOP) Picture Layer Slice Layer Macroblock Layer Block Layer ______________________________________
The top four layers of the coded bit stream contain only fixed length codes which generally describe the picture structures, code book assignments for decoding, and some of the global compression decisions. The number of code words in these four layers does not vary significantly and is also independent of the coding decision. Starting with the Macroblock layer however, variable length codes are used and the number and length of code words vary therefore with the coding decision made by the encoder. Most compression of a picture is achieved in the Macroblock and Block layers and therefore, another object of the invention is to provide a decoder which will process a bitstream containing variable length codes in a manner which will permit maximum use of decompression hardware and software.